prefer-object-spread: autofixer may produce invalid code

[ajafff]

Related: #186

obj = Object.assign({prop: 1}, {prop: 2});

// fixed to
obj = {prop: 1, prop: 2}; // duplicate key 'prop'

Produces duplicate key error. Found this in the test cases of the rule...

Possible fixes:

• try really hard to work around the error
  • not fixing such cases
  • removing all overridden occurrences of the duplicate property
    • may introduce even more changes if the initializer has side effects
• fix to object spread and let no-useless-object-spread do it's job (which is currently not autofixing)
• don't fix at all
• make this fix a "suggested / unsafe fix" (introduce suggested fixes for potentially unsafe or undesired fixes #207)

[ajafff]

Closing as "won't fix" for the reasons outlined in #249 (comment)